Methods of making sintered articles

ABSTRACT

Methods of making sintered articles from powder metal carbide compositions by additive manufacturing techniques are described herein. Sintered carbide articles fabricated by such additive manufacturing techniques, in some embodiments, exhibit densities equaling articles formed according to conventional techniques employed in powder metallurgy. For example, a method of manufacturing an article comprises providing sintered cemented carbide powder comprising a hard particle phase including tungsten carbide and a metallic binder phase and forming the sintered cemented carbide powder into a green article by one or more additive manufacturing techniques. The green article is sintered to provide a sintered article having density greater than 90% theoretical full density, wherein the green article has a density less than 50% theoretical full density prior to sintering.

FIELD

The present invention relates to sintered articles and, in particular,to methods of making sintered articles from powder metal carbidecompositions by additive manufacturing techniques.

BACKGROUND

Additive manufacturing generally encompasses processes in which digital3-dimensional (3D) design data is employed to fabricate an article orcomponent in layers by material deposition and processing. Severaltechniques have been developed falling under the umbrella of additivemanufacturing. Laser sintering, for example, is a common additivemanufacturing technique wherein a thin layer of powder material isapplied to a building substrate or platform. A laser beam subsequentlyfuses the powder at points predetermined by the digital data encodingthe shape and dimensions of the article to be fabricated. The platformis then lowered and another layer of powder is applied and selectivelyfused to bond with the layer below at the predetermined points. Thisprocess is repeated until fabrication of the article is complete.

In view of this example, additive manufacturing offers an efficient andcost-effective alternative to traditional article fabrication techniquesbased on molding processes. With additive manufacturing, the significanttime and expense of mold and/or die construction and other tooling canbe obviated. Further, additive manufacturing techniques make anefficient use of materials by permitting recycling in the process andprecluding the requirement of lubricants and coolant. Most importantly,additive manufacturing enables significant freedom in article design.Articles having highly complex shapes can be produced withoutsignificant expense allowing the development and evaluation of a seriesof article designs prior to final design selection.

SUMMARY

Methods of making sintered articles from powder metal carbidecompositions by additive manufacturing techniques are described herein.Sintered carbide articles fabricated by such additive manufacturingtechniques, in some embodiments, exhibit densities and hardness equalingarticles formed according to conventional powder metallurgicaltechniques. For example, a method of manufacturing an article comprisesproviding sintered cemented carbide powder comprising a hard particlephase including tungsten carbide and a metallic binder phase and formingthe sintered cemented carbide powder into a green article by one or moreadditive manufacturing techniques. The green article is sintered toprovide a sintered article having density greater than 90% theoreticalfull density, wherein the green article has a density less than 50%theoretical full density prior to sintering. In some embodiments, thedensity of the sintered article is greater than 95% theoretical fulldensity.

In another embodiment, a method of manufacturing an article comprisesproviding a sintered cermet powder having a hard particle phaseincluding at least one of a carbide, nitride and carbonitride of a GroupIVB metal and a metallic binder phase. The sintered cermet powder isformed into a green article by one or more additive manufacturingtechniques. The green article is sintered to provide a sintered articlehaving density greater than 90% theoretical full density, wherein thegreen article has a density less than 50% theoretical full density priorto sintering. In some embodiments, the density of the sintered articleis greater than 95% theoretical full density.

These and other embodiments are further described in the detaileddescription which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a scanning electron microscopy (SEM) image of sinteredcemented carbide powder according to one embodiment described herein,and FIG. 1(b) is a cross-sectional SEM image of sintered cementedcarbide particles of FIG. 1(a).

FIG. 2 is a cross-sectional SEM image of sintered cemented carbidepowder according to one embodiment described herein.

FIG. 3 is an X-ray diffractogram of sintered cemented carbide powderaccording to one embodiment described herein.

FIG. 4 is a cross-sectional metallographic image (i.e. of the xy-plane)of a sintered and hot isostatic pressed article fabricated according toa method described herein.

FIG. 5(a) is an SEM image of sintered cemented carbide powder accordingto one embodiment described herein, and FIG. 5(b) is a cross-sectionalSEM image of sintered cemented carbide particles of FIG. 5(a).

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by referenceto the following detailed description and examples and their previousand following descriptions. Elements, apparatus and methods describedherein, however, are not limited to the specific embodiments presentedin the detailed description and examples. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Numerous modifications and adaptations will bereadily apparent to those of skill in the art without departing from thespirit and scope of the invention.

Methods of making sintered articles from powder metal carbidecompositions by additive manufacturing techniques are described herein.A method of manufacturing an article comprises providing sinteredcemented carbide powder comprising a hard particle phase includingtungsten carbide and a metallic binder phase and forming the sinteredcemented carbide powder into a green article by one or more additivemanufacturing techniques. The green article is sintered to provide asintered article having density greater than 90% theoretical fulldensity, wherein the green article has a density less than 50%theoretical full density prior to sintering.

Turning now to specific components, methods described herein employsintered cemented carbide powder comprising a hard particle phaseincluding tungsten carbide and a metallic binder phase. Tungsten carbideof the hard particle phase can consist of stoichiometric WC. FIG. 3, forexample, is an X-ray diffractogram of a sintered cemented carbide powderwherein WC is the sole phase present. Alternatively, tungsten carbide ofthe hard particle phase can comprise a WC phase and W₂C phase. Further,the hard particle phase can be formed solely of tungsten carbide. Inother embodiments, the hard particle phase can further comprise carbide,nitride and/or carbonitride of one or more metals selected from GroupsIVB-VIB of the Periodic Table. For example, in addition to tungstencarbide, the hard particle phase can include at least one of tantalumcarbide, niobium carbide, vanadium carbide, chromium carbide, zirconiumcarbide, hafnium carbide, titanium carbide and solid solutions thereof.

As described herein, the sintered cemented carbide powder also includesa metallic binder phase. In some embodiments, the metallic binder phaseis formed of cobalt, nickel or iron or alloys thereof. The metallicbinder phase can be present in the sintered cemented carbide powder inany amount not inconsistent with the objectives of the presentinvention. Generally, the metallic binder phase is present in an amountof 3 wt. % to 30 wt. % of the sintered cemented carbide powder. In someembodiments, the metallic binder phase is present in an amount selectedfrom Table I.

TABLE I Metallic Binder Phase Wt. % of Sintered Cemented Carbide Powder 5-25 10-20 10-30 15-25

In being sintered, an individual particle of the cemented carbide powderis formed of smaller particles bound together by the metallic binderphase. FIG. 1 illustrates this microstructure in a sintered WC-17% Copowder. As provided in FIG. 1(a), WC particles are bound by Co binder toform individual sintered cemented carbide particles. FIG. 1(b) providesa cross-sectional image of the sintered cemented carbide particles whereCo binder phase is differentiated from the WC particle phase. FIG. 2also provides a cross-sectional SEM image of a sintered WC-20% Co powderwhere metallic binder phase is differentiated from hard particle phase.Hard particles, including tungsten carbide particles, can be intensivelymilled with powder metallic binder and subsequently spray dried. Theresulting hard particles and associated binder are sintered to providethe sintered cemented carbide powder.

The sintered cemented carbide powder can have any desired averageparticle size. For example, the sintered cemented carbide powdergenerally has an average particle size of 0.1 μm to 100 μM. Averageparticle size of the sintered cemented carbide powder can be selectedaccording to several considerations including desired density andhardness of the sintered article formed from the powder, packingcharacteristics of the powder and compatibility of powder flowcharacteristics with the additive manufacturing technique employed ingreen forming. In some embodiments, the sintered cemented carbide powderhas an average particle size selected from Table II.

TABLE II Average Particle Size of Sintered Cemented Carbide Powder (μm) 1-50  5-45 10-30 20-50

One or more surface treatments can be applied to the sintered cementedcarbide powder. Surface treatments can be applied to alter and/orenhance packing and flow characteristics of the sintered cementedcarbide powder. Suitable surface treatments, in some embodiments,comprise waxes, polymeric species and/or other organic dispersantspecies.

Moreover, powder metal carbide, metal nitride and/or metal carbonitridecan be added to the sintered cemented carbide powder. Additional powdermetal carbide, nitride and/or carbonitride can be added in any desiredamount. In some embodiments, for example, additional powder metalcarbide, nitride and/or carbonitride can be added in an amount of 0.1 to5 wt. % of the sintered cemented carbide powder. The additional powdermetal carbide, nitride and/or carbonitride can exhibit an averageparticle size commensurate with average particle size of the sinteredcemented carbide powder. Alternatively, the additional powder metalcarbide, nitride and/or carbonitride can have an average particle sizeless than the sintered cemented carbide powder. In such embodiments, theadditional powder may fill gaps or interstitial spaces between sinteredcemented carbide particles. Metal of the additional powder carbide,nitride and/or carbonitride can be one or more transition metals,including transition metals selected from Groups IIIB-VIIIB of thePeriodic Table.

As described herein, the sintered cemented carbide powder is formed intoa green article by one or more additive manufacturing techniques. Anyadditive manufacturing technique operable to form the sintered cementedcarbide powder into a green article having properties described hereincan be employed. In some embodiments, additive manufacturing techniquesemploying a powder bed are used to construct green articles formed ofsintered cemented carbide powder. For example, binder jetting canprovide a green article formed of sintered cemented carbide powder. Inthe binder jetting process, an electronic file detailing the designparameters of the green part is provided. The binder jetting apparatusspreads a layer of the sintered cemented carbide powder in a build box.A printhead moves over the powder layer depositing liquid binderaccording to design parameters for that layer. The layer is dried, andthe build box is lowered. A new layer of sintered cemented carbidepowder is spread, and the process is repeated until the green article iscompleted. In some embodiments, other 3D printing apparatus can be usedto construct the green part from the sintered cemented carbide powder inconjunction with organic binder.

The green article fainted of the sintered cemented carbide powder issintered to provide a sintered article having density greater than 90%theoretical full density. In some embodiments, density of the sinteredarticle is greater than 95% or 97% theoretical full density. Asdescribed herein, the green article exhibits density of less than 50%theoretical full density prior to sintering. In some embodiments, thegreen article has a density selected from Table III.

TABLE III Green Article Density % Theoretical Full Density ≤45 ≤40 ≤3010-40 20-30The high density of the sintered article formed from the low densitygreen article is unexpected. General knowledge in the art requires greenparts formed of cemented carbide powder to have at least 50% theoreticalfull density for proper sintering and attainment of acceptable sintereddensity. Conventional powder metallurgical techniques employ pressingoperations to sufficiently densify cemented carbide powder compositionsprior to sintering. As detailed in the examples herein, low densitygreen parts formed of sintered cemented carbide powder can be providedby additive manufacturing techniques and sintered to high densitywithout pressing or other densification operations.

Green articles described herein can be sintered under conditions and fortime periods to provide sintered articles having the desired density.The green part can be vacuum sintered or sintered under a hydrogen orargon atmosphere at temperatures of 500° C. to 2000° C. In someembodiments, the sintering temperature is 1300° C. to 1560° C. Moreover,sintering times can range from 10 minutes to 10 hours. In someembodiments, hot isostatic pressing (HIP) is added to the sinteringprocess. Hot isostatic pressing can be administered as a post-sinteroperation or during vacuum sintering. Hot isostatic pressing can beadministered for up to 10 hours at pressures of 1 MPa to 300 MPa andtemperatures of 800° C. to 2000° C. Sintered articles described hereinthat are subjected to HIP can exhibit densities greater than 98%theoretical full density. In some embodiments, density of a sintered-HIParticle is at least 99% theoretical full density.

Sintered articles described herein can have hardness of 500 to 3000HV500 gf. HV500 gf refers to Vickers Hardness using a 500 gram-forceload. The microhardness equipment is certified according to ASTM E384—Standard Methods for Knoop and Vickers Hardness Materials. In someembodiments, for example, a sintered article has hardness of 700-1500HV30. Additionally, sintered articles described herein can be free orsubstantially free of lower carbide phases, including eta phase[(CoW)C], W₂C and W₃C. Alternatively, the sintered articles can includelower carbide phases in minor amounts (generally <5 wt. %). Moreover, asintered article described herein can have an average grain size lessthan 100 μm. In some embodiments, for example, a sintered article has anaverage grain size of 1-50 μm or 10-40 μm.

In another embodiment, a method of manufacturing an article comprisesproviding a sintered cermet powder having a hard particle phaseincluding at least one of a carbide, nitride and carbonitride of a GroupIVB metal and a metallic binder phase. The sintered cermet powder isformed into a green article by one or more additive manufacturingtechniques. The green article is sintered to provide a sintered articlehaving density greater than 90% theoretical full density, wherein thegreen article has a density less than 50% theoretical full density priorto sintering. In some embodiments, density of the sintered article isgreater than 95% or 97% theoretical full density.

Hard particle phase of the sintered cermet powder, in some embodiments,comprises at least one to TiC, TiN and TiCN. Further, the metallicbinder phase can be selected from the group consisting of nickel,cobalt, molybdenum and alloys thereof. The sintered cermet powder can besimilar in structure to sintered cemented carbide powders describedherein. For example, an individual particle of the cermet powder isformed of smaller hard particles bound together by the metallic binderphase. The sintered cermet powder can have an average particle size of0.1 μm to 100 μm.

Sintered cermet articles can have properties, including density,hardness and average grain size, commensurate with the sintered cementedcarbide articles described herein. Moreover, green articles formed ofsintered cermet powder can have densities selected from Table III above.Such green articles can be sintered and processed under conditionsdescribed above.

Sintered articles produced according to methods described herein can beemployed in a variety of industries including petrochemical, automotive,aerospace, industrial tooling and manufacturing. In some embodiments,the sintered articles are used as components exposed to wearenvironments or abrasive operating conditions such as bearings, valvesand/or fluid handling components.

These and other embodiments are further illustrated by the followingnon-limiting examples.

Example 1—Sintered Article

Sintered cemented carbide powder (WC-17% Co) was loaded into the chamberof a 3D printing system from ExOne of N. Huntingdon, Pa. The 3D printingsystem generated the green article in a layered process using iterativespreading of the WC-17% Co powder in a build box followed by applicationof binder according to the design of the three-dimensional article. TheWC-17% Co powder possessed a microstructure as illustrated in FIGS.1(a)-(b) and was free of any lower carbide phases as evidenced by thediffractogram of FIG. 3. The completed green article exhibited densityof about 27.3% theoretical full density. The green article was removedfrom the 3D printing chamber and vacuum sintered at a temperature ofabout 1480° C. for 45 minutes followed by fast cooling. The resultingsintered article exhibited density of 97.7±0.4% theoretical fulldensity. HIP was subsequently administered to the sintered article for70 minutes at a temperature of about 1425° C. and pressure of 20,000±500psi. Density of the sintered article increased to 98.7±0.2% theoreticalfull density after HIP. FIG. 4 is a cross-sectional metallographic imageof the sintered article. Further, hardness of the sintered article wasdetermined to be 1100 HV500 gf. Such hardness was commensurate with asintered article of WC-17% Co formed according to conventional powdermetallurgical techniques as provided in Table IV.

TABLE IV Hardness Comparison (HV500gf) WC-17% Co Example 1 WC-17% CoPowder Metallurgy 1100 1050

Example 2—Sintered Article

A sintered article was produced according to the procedure set forth inExample 1, the difference being the use of sintered cemented carbidepowder of WC-20% Co. A cross-sectional SEM image of the sintered WC-20%Co powder is provided in FIG. 2. Similar to Example 1, density of thegreen article was also less than 30% theoretical full density and thefinal density of the sintered article following HIP was 96.3%theoretical full density.

Example 3—Sintered Cemented Carbide Powder

WC-12% Co powder was produced by spray drying and sintering. Thissintered cemented carbide powder can be used to fabricate sinteredarticles according to methods described herein. SEM images of the WC-12%Co powder are provided in FIGS. 5(a)-(b).

Various embodiments of the invention have been described in fulfillmentof the various objects of the invention. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Numerous modifications and adaptations thereof willbe readily apparent to those skilled in the art without departing fromthe spirit and scope of the invention.

The invention claimed is:
 1. A method of manufacturing an articlecomprising: providing sintered cemented carbide powder consisting oftungsten carbide, and a metallic binder phase; forming the sinteredcemented carbide powder into a green article by one or more additivemanufacturing techniques, the green article consisting essentially ofthe sintered cemented carbide powder and having density less than 50%theoretical density; and sintering the green article having density lessthan 50% theoretical density to provide a sintered article havingdensity greater than 90% theoretical full density.
 2. The method ofclaim 1, wherein the density of the sintered article is greater than 95%theoretical full density.
 3. The method of claim 1, wherein the densityof the green article is less than 40% theoretical full density prior tosintering.
 4. The method of claim 1, wherein the density of the greenarticle is less than 30% theoretical full density prior to sintering. 5.The method of claim 1, wherein the tungsten carbide consists of a WCphase.
 6. The method of claim 1, wherein the tungsten carbide comprisesa WC phase and W₂C phase.
 7. The method of claim 1, wherein the metallicbinder phase comprises at least one of cobalt, nickel and iron.
 8. Themethod of claim 7, wherein the metallic binder phase is present in anamount of 3 wt. % to 30 wt. % of the sintered cemented carbide powder.9. The method of claim 1, wherein the sintered cemented carbide powderhas an average grain size of 0.1 μm to 100 μm.
 10. The method of claim1, wherein the sintered cemented carbide powder has an average grainsize of 1 μm to 50 μm.
 11. The method of claim 1, wherein the sinteredarticle is free of lower carbide phases.
 12. The method of claim 11,wherein the lower carbide phases include eta phase, W₂C and W₃C.
 13. Themethod of claim 1, wherein tungsten carbide of the sintered articlecomprises a WC phase and W₂C phase.
 14. The method of claim 1, whereinthe sintered article has an average grain size less than 100 μm.
 15. Themethod of claim 1, wherein the sintered article has hardness of 500 to3000 HV500gf.
 16. The method of claim 1, wherein the green article issintered at a temperature of 500° C. to 2000° C.
 17. The method of claim1 further comprising hot isostatic pressing the sintered article. 18.The method of claim 17, wherein the hot isostatic pressing isadministered during sintering.
 19. The method of claim 17, wherein thesintered article has a density greater than 98% theoretical fulldensity.
 20. The method of claim 1, wherein the additive manufacturingtechnique employs the sintered cemented carbide powder as a powder bedfrom which the green article is formed.
 21. The method of claim 20,wherein the additive manufacturing technique is binder jetting.